Research And Improvement On Physical Layer And MAC Layer Protocol In Vehicular Communication System

In recent years,with the improvement of social production and the development of automobile industry,the ownership of automobiles has increased rapidly.While bringing convenience to people,automobiles have also caused many problems such as road congestion,traffic accidents,and environmental pollution.Therefore,people pay more and more attention to intelligent transportation,and vehicular communication system which combines wireless communication technologies,sensor networks and computer principles is becoming research hotspots in many countries.Vehicular communication is based on a variety of different communication protocols.At present,there are no uniform standards in the world,which there are two mainstreams,one is the DSRC standard for short-range communications represented by the United States and the other is the LTE-V2X standard based on cellular communications represented by China.This article mainly focuses on the vehicular communication system based on DSRC standard.Under the DSRC protocol architecture,the IEEE 802.11p standard is used as the physical layer and the MAC layer protocol,and the IEEE 1609.4 standard is used to support multi-channel operations,which is called enhanced MAC protocol.The physical layer provides transmission and reception of data between users and determines the quality of the communication.Meanwhile,the MAC layer coordinates the node access channel resources in the network to determine whether the network can operate efficiently.The physical layer and the MAC layer are closely linked and cooperate with each other to jointly constitute the technical basis of the vehicular communication system.The article pays attention to the technology issues in vehicular communication and conducts in-depth research on physical layer and MAC layer's communication protocols and key technologies.The main work and innovative results are as follows:(1)More appropriate channel estimation and channel equalization algorithms are studied.Since there is a close relationship between synchronization estimation and channel estimation in vehicular OFDM system,the improved algorithm considers joint estimation of both at the receiving end,so as to eliminate the ICI caused by carrier frequency offset as much as possible.Meanwhile,an adaptive gradient-varying channel estimation algorithm is adopted based on principle of decision feedback.Compared with the traditional algorithm,the channel estimation of the proposed algorithm is obviously more accurate,and the system gain is larger,especially when the Doppler frequency shift is high.(2)The LDPC code is introduced into the vehicular communication system and an improved decoding algorithm is proposed.A key technology of LDPC codes is iterative decoding using belief propagation,thus bringing a lot of advantages on performance and security over Turbo codes.The LDPC decoding algorithm based on the principle of stochastic computation can present many computational processes in the form of logical operations.The improved tracking forecast memory decoding algorithm and its derived means can realize the accurate transmission of node information,and avoid the occurrence of latching phenomenon,thereby effectively reducing the system hardware and software complexity without losing much performance.(3)Combining the TDMA mechanism with the principle of cooperative communication,an improved MAC layer protocol scheme is proposed.Considering the requirements for the transmission of safety information and non-safety information during vehicular communication,the proposed clustering-based MAC protocol allocates two sub-slots to each node of the network to effectively reduce the time delay and improve the channel utilization.At the same time,cluster-based multi-channel cooperative retransmission mechanism can more effectively use relay nodes to increase the probability of successful message reception.Simulation results of various parameters prove that the improved MAC layer protocol can make full use of the channel and time slot resources to provide better QoS guarantee for the vehicular communication system.The full text starts from the physical layer,which is the lowest layer of the vehicular communication system protocol architecture,and goes up to the MAC layer,and then to the multi-channel operation of the MAC layer.It also understands the different transmission requirements for security and non-security applications in the vehicular communication process.The research on protocols and key technologies provides suggestions for the future development of vehicular communication systematically and comprehensively.